In Japan, a medical interest in aging is increasing as an aging society is being approached. Accordingly, the mechanism of skin aging and other subjects in the field of dermatology are under investigation, and the control of aging by cell potentiator administration is drawing much attention both in Japan and abroad.
Traditionally, some animal, plant or fungal extracts have been used in pharmaceuticals and cosmetics, with the recognition of the tissue potentiating activity thereof. However, few of them have been assessed as exhibiting bioactivity at cell level. For example, attempts have been made to determine the increase in oxygen consumption caused by the addition of these extracts to tissue sections using Warburg's manometer, etc., or to determine the tissue potentiating activity using an enzyme which catalyzes oxidative phosphorylation for ATP, etc. However, in these means, for example, the oxygen consumption determination method using tissue sections shows that water as well possesses potentiating activity; even it can be said that many substances containing a carbon or nitrogen source, at appropriate concentrations, increase oxygen consumption. Although these means are effective at least in the evaluation of tissue potentiators as nutritive agents, there have been many difficulties in assessing cell potentiating activity by these means, provided that samples are evaluated from the viewpoint of skin function at cell level.
Histochemically, the skin can be roughly divided into the epidermis, the dermis and the subcutaneous tissue. Particularly the dermis, as a skin supporting tissue, plays a key role in the maintenance of skin homeostasis. The dermis is configured mainly with fibroblasts, which produce proteins such as collagen and glycosaminoglycans such as hyaluronic acid to form a constructive structure in these connective tissues.
The major glycosaminoglycan present in the skin dermal tissue is hyaluronic acid, a high molecular compound of a molecular weight of up to several millions having repeat units comprising glucuronic acid and N-acetylglucosamine bound via .beta.-glycoside bond. Hyaluronic acid is also abundantly contained in chicken crests, umbilical cords, ocular glass and secretions such as joint fluid. Also, since hyaluronic acid has a very long acidic sugar chain, it is expected to have the action as a kind of polyanion, with high water retention capability; 1 g of hyaluronic acid can retain as much of 6 liters of water. Physiologically, it can therefore be conjectured that skin flexibility may be closely associated with the function of this substance.
In recent years, various studies of aging have been conducted, and hyaluronic acid has proven to play a key role in skin function. Various cosmetics have been formulated with hyaluronic acid derived from chicken crest or Streptococcal microbial fermentation to utilize the excellent functions of hyaluronic acid, such as skin moisture retention. However, these cosmetics remain on the skin surface and merely exhibit moisture retaining action attributable to the water absorbing property thereof, since they offer nothing more than skin surface application of hyaluronic acid and since the absorption of hyaluronic acid, as a high molecular substance, is hampered by the epidermal barrier. The effect disappears upon washing down the cosmetic; such conventional cosmetics do not offer essential improvement in skin function.
Hyaluronic acid, playing a key role in skin function, has been reported to decrease with aging. Miyamoto et al. investigated the relationship among hyaluronic acid, skin moisture content and aging, using the rat skin, and found that hyaluronic acid had very high values in the fetal and neonatal stages and decreased rapidly until 4 weeks of age, followed by gradual decrease. It was also shown that the hyaluronic acid decreasing pattern and the dry yield pattern are in a reverse correlation, accordingly the dermal water content depends strongly on hyaluronic acid (J. Soc. Cosmet. Chem. Japan, 15, 77, 1981).
In a study using fibroblasts derived from human fetal lung, Matsuoka et al. showed that glycosaminoglycan synthesis decreases with the number of subculture generations, and that hyaluronic acid decreases markedly (Cell Structure and Function, 9, 357, 1984).
Also, Maria et al. determined hyaluronic acid contents in normal human female skin and demonstrated a reduction in skin hyaluronic acid content with aging (Carbohydrate Research, 159, 127-136, 1987).
From these findings, it is evident that hyaluronic acid synthesis in skin fibroblasts decreases with aging. This suggests a potential for suppression of morphological changes in aged skin, such as wrinkles, by increasing the dermal hyaluronic acid content and hence providing flexibility and smoothness for the skin, provided that hyaluronic acid synthesis can be promoted.